PROJECT SUMMARY This project is submitted under Funding Opportunity Announcement (FOA) Number: RFA-DA-19-002. Opioids have been significantly over-prescribed and are associated with numerous deaths, resulting in the Nation?s current opioid crisis. The FDA recently approved the ?2 adrenergic agonist lofexidine as a non-addictive, non-opioid treatment for opioid use disorder. This preclinical drug development effort stems from the psychoactive, natural product, Mitragyna speciosa (kratom), a Thai medicinal herb used as a self-treatment for opioid use disorder. Mitragynine, the plant?s most abundant alkaloid, is a low efficacy receptor agonist with G- protein signaling bias. Our preliminary studies suggest that mitragynine has limited abuse liability, and interacts with non-opioid CNS targets including ?2 adrenergic receptors, which have not been exploited in its unique mechanism. A single drug (mitragynine) that interacts with both opioid and ?2 adrenergic receptors would offer a highly innovative approach for treating opioid use disorder. The work planned here, involving a collaborative, interdisciplinary team, will examine the pharmacophoric elements of mitragynine through synthetic derivatives in an approach that led to the understanding of the essential pharmacophore of morphine. We will use a combination of chemical and prodrug synthesis, in vitro metabolic stability, affinity and efficacy analysis, behavioral assays predictive of receptor mechanism (drug discrimination), abuse (self-administration), and untoward effects (respiratory depression, tolerance, and dependence), and in vivo ADME assays. Mitragynine analogs are expected to yield innovative compounds with a pharmacological mechanism that includes opioid and adrenergic activity. Our efforts to identify the pharmacophoric requirements of mitragynine will lead to templates for the design of novel opioid receptor ligands; this will greatly improve the knowledge of interactions of these structurally novel compounds with opioid receptors and facilitate the development of these ligands as treatments for opioid use disorders. The specific aims of the 2-year UG3 phase are as follows. AIM 1: Identify opioid pharmacophoric requirements of mitragynine analogs through deletion design and analog stability; identify mitragynine prodrugs. AIM 2: Investigate mitragynine analogs in drug discrimination, self-administration, and respiration assays. Analogs exhibiting desired metabolic stability, bioavailability, blood-brain-barrier penetration, binding characteristics, and behavioral activity will be further studied in the UH3 phase as follows. AIM 3: Establish comprehensive in vivo ADME of mitragynine analogs and prodrugs. AIM 4: Assess mitragynine analogs and prodrugs in tolerance, dependence, and withdrawal assays. The results of this project will provide a more comprehensive understanding of the chemical requirements of the putative recognition elements of mitragynine-related ligands at opioid and ?2 adrenergic receptors. Ultimately, the potential use of mitragynine and its analogs as templates for the development of a new treatment for opioid use disorders will be realized that may have the potential to yield a safe, effective FDA-approved pharmacotherapy.